1. Field of the Invention
The present invention relates to a planar light emitting device and a method of manufacturing thereof, in which the planar light emitting device is used for the illumination of operation buttons or key buttons of a mobile phone, a personal digital assistant (PDA), a personal computer and the like.
2. Description of the Related Art
When operating the operation buttons or key buttons of mobile phones, PDAs, and personal computers and the like in dark places, these buttons or keys are illuminated in order to make the positions of the operation buttons and key buttons easily viewable.
Conventionally, in order to brightly illuminate the key buttons of a mobile phone and the like, a planar light emitting device is used, and as one aspect of this planar light emitting device, a side surface light emitting type planar light emitting device is widely employed.
That is to say, this side surface light emitting type planar light emitting device is constituted from a light guiding plate (light guiding film) that is arranged on the back surface side of the display panel such as a liquid crystal panel, and a light source that is arranged at a side end face of this light guiding film. Also, examples of the light source that is applied to this planar light emitting device include a light emitting diode (LED), a cold-cathode tube, and the like.
As one example of this illumination device, for example, Patent Document 1 discloses a push button switch-type illumination device. As shown in FIG. 11, in this push button switch-type illumination device, a substrate 103 is arranged inside of a case that consists of an upper case 101 and a lower case 102. A keypad 105 that is provided with a plurality of operation keys 104 is provided above this substrate 103. A light guiding plate 106 is arranged between this keypad 105 and the substrate 103, and switching elements 107 corresponding to the operation keys 104 are arranged on the upper surface side of the substrate 103. The operation keys 104 of the keypad 105 are exposed to the outside via a plurality of operation holes 101a that are formed in the upper case 101. Along with that, a display device 109 such as a liquid display panel is provided at an end portion of the substrate 103. This display device 109 can be viewed from outside via a display window 101b that is formed in the upper case 101.
Then, the light guiding plate 106 provided between the substrate 103 and the keypad 105 illuminates the operation keys 104 from the rear-face side (inner side). A light source 108 is provided at one end portion of the light guiding plate 106. The light guiding plate 106 projects the light that is made incident from the light source 108 onto the light guiding plate 106 to the rear-face side of the keypad 105, and thereby illuminates the operation keys 104 from the rear-face side.
Also, as another structure example of a planar light emitting device that is provided with this light guiding plate, a plurality of the light sources 111 are arranged at the end portion side of the light guide 110, as shown in FIG. 12 and FIG. 13. A switch sheet 112 is arranged on the rear-face side (the lower-surface side in the drawing) of the light guide 110. A key mat 113 is arranged on the front-surface side (the upper-surface side in the drawing) of the light guide 110. A light extracting portion 110a that consists of a plurality of concave-convex portions (concave-convex portions) is provided at required positions of the light guide 110, and illumination light is guided to the surface side (upper-face side) of the light guide 110 via the light extracting portion 110a. 
In recent years, in this kind of planar light emitting device, it has been desired to illuminate the operation keys so as to selectively cause only specified operation keys to emit light, or so as to selectively cause only specified operation keys not to emit light.
As a method of achieving this issue, for example Patent Document 2 discloses a structure that provides a light-blocking portion that consists of a non-light-transmissive resin at the light guide. That is to say, in this structure, by dividing the aforementioned light guide into a plurality of regions, providing a slit at the portion that becomes the boundary of each region, and filling the non-light-transmissive resin in the interior of the slit, the light-blocking portion that consists of the non-light-transmissive resin is provided at the boundary of each region of the light guide. In order to realize this structure, in the structure disclosed in Patent Document 2, as shown in FIG. 14A to FIG. 14D, a UV curable resin film 151 is formed by spin coat method on a carrier base 150, and after curing the resin film, a slit 152 is formed at the required position by selective etching. Thereafter, using a resin dispenser 153, a black resin 155 is filled in the slit 152, and by curing this resin 155, the light-blocking portion 156 is formed.
Also, for example Patent Document 3 discloses an illumination device that can respectively illuminate the upper portion and lower portion of a push button using only one light guide. This illumination device is provided with a light-blocking rib that has a notched portion, and a light guide body that is attached to this light-blocking rib. This light guide body is provided with a first light guide portion and a second light guide portion that illuminate the upper portion and lower portion of the push button respectively. These first and second light guide portions are integrated via a crank-shaped connection portion. By fitting the crank-shaped connection portion in the notched portion of the light-blocking rib, the light guide body is attached to the light-blocking rib. In this way, by using only one light guide body, it is possible to perform separate illumination.
Furthermore, for example Patent Document 4 (Japanese Unexamined Patent Application, First Publication No. 2008-152951) discloses a switch sheet module that can cause a plurality of key switches to emit light with fewer light emitting elements at every group. This switch sheet module is provided with a light guide sheet portion that covers the upper surface of the key switch, and the light guide sheet portion is constituted by a plurality of light conductive sheet materials of which at least one portion is overlapped, and an LED is arranged at each light conductive sheet material.
The method that is disclosed in paragraph 0034 to paragraph 0035 of Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2008-41431) manufactures the UV-curable resin film 151 that becomes a light guide by spin coat method (as shown in FIG. 14A to FIG. 14C), and after subjecting this resin film 151 to UV curing, forms the slit 152 by selective etching. Thereafter, resin is injected into this slit 152, and by curing this resin, a hard light-blocking portion 156 is formed. Accordingly, the method according to Patent Document 2 has the problem that the manufacture requires excessive time and effort.
Also, as shown in paragraph 0036 to paragraph 0037 of Patent Document 2, even in the case of adopting a method that applies an insert formation technique, since a step that sandwiches a black, hard resin in a metal mold and flows a light guide material from both sides is required, it has the problem of requiring excessive time and effort for manufacture.
The method that is disclosed in Patent Document 2 is a method that provides the light-blocking portion 156 in the resin film 151 that is formed with the spin coat method and has a thickness of about 0.3 mm to 2 mm, and the thickness of the light-blocking portion 156 that is filled and formed in the slit 152 of the resin film 151 is 0.3 mm to 2 mm, and thus extremely thick. As shown in FIG. 5 and FIG. 6 of Patent Document 2, the light-blocking portion 156 and the resin film 151 that are integrally molded are completely bonded. Accordingly, in the case of the material that constitutes the light-blocking portion 156 differing from the material that constitutes the resin film 151, there is a possibility of the light-blocking portion 156 exfoliating by a change in the environmental temperature, due to the difference of the linear expansion coefficients of these materials. Also, by having a structure that fills in the entirety of the slit 152 with the light-blocking portion 156, the heat dissipation around the slit formation portion of the resin film 151 worsens, and so there is the possibility of discoloration occurring in the resin material that constitutes the light guide due to the working temperature environment.
Furthermore, it is conceivable to apply the technique disclosed in Patent Document 3 (Japanese Unexamined Utility Model Application, First Publication No. H05-53070) to provide a light-blocking member in a push button switch-type illumination device disclosed in Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2001-167655). However, the field of application of the technique given in Patent Document 3 is car audio, and the structure of Patent Document 3 is therefore can be applied when there is ample space. On the other hand, in the field of mobile telephones, there is a desire to make the overall thickness as thin as possible, and so applying the technique that is disclosed in Patent Document 3 is not easily done.